Direct evidence for the partial reflection of hyaluronan molecules by the lining of rabbit knee joints during trans-synovial flow.

1. In synovial joints hydraulic and turnover studies indicate that the synovial lining may partially reflect large macromolecules like hyaluronan, despite discontinuities in the lining cell layer. The reflection hypothesis was tested directly in the present study. 2. Solutions of high molecular weight hyaluronan were infused at controlled pressures into the cavity of rabbit knees under anaesthesia, at concentrations of 0.2 g l-1 (n = 5), 2 g l-1 (n = 5) and 4 g l-1 (n = 6). Time-averaged trans-synovial flows were 9.6, 4. 8 and 2.9 microl min-1, respectively. After 5 h infusion the intra-articular fluid was mixed and sampled. Hyaluronan concentration was determined by size-exclusion chromatography. 3. In all sixteen experiments the hyaluronan concentration in the aspirate was greater than that in the infusate (P = 0.0001, Student's paired t test). The increases averaged 2.28 +/- 0.04 times at high filtration rates (0.2 g l-1 infusates; mean +/- S.E.M.), 1.60 +/- 0. 09 times at intermediate filtration rates (2 g l-1 infusates) and 1. 26 +/- 0.08 times at low filtration rates (4 g l-1 infusates). 4. Between 48 and 95 % of the hyaluronan in the filtrand was retained in the joint cavity. The greater retention at 2 g l-1, viz.95 %, than at 0.2 g l-1, viz.48 %, was attributed to interactions between overlapping molecular domains in the more concentrated solution. 5. It is concluded that synovial interstitial matrix can partially reflect hyaluronan molecules, and thus conserve intra-articular lubricant.